Efficacy of anthelmintics in horses and donkeys in Ireland: an in vivo and in vitro study

Strongyles are the most important parasite group infecting equids. Management of these parasites has relied on intensive use of anthelmintics, however, resistance has developed against all drug classes and is becoming a major practical problem in many countries. Resistance to the benzimidazole (BZ) group is geographically widespread and resistance to pyrental has also been reported. Today the macrocyclic lactones (ML) class of drugs has become the most commonly used drug, but evidence of emerging resistance (i.e. shortened egg reappearance period (ERP)) has been identified in many countries. A variety of tests are available to monitor anthelmintic efficacy but most of them are expensive, laborious and time consuming. The aim of this project was to determine the efficacy of anthelmintic drugs used in eight equine groups in Ireland. The anthelmintic efficacy was determined by calculating the percentage reduction in the faecal egg count (FEC) between the group mean at Day 0 and Day 14 post-treatment (FECRT). In addition FECs were also calculated at two week intervals for up to 16 weeks after anthelmintic drug administration to determine the ERP for BZ, ivermectin and moxidectin. ERP was defined when the group arithmetic mean FEC exceeded 10% of the group arithmetic mean FEC at Day 0.The larval development assay (LDA) was used to detect resistance to BZ in two groups of horses and the larval migration inhibition assay (LMIA) was also performed to measure the sensitivity to ivermectin in two groups of donkeys and moxidectin in two horse farms and two groups of donkeys. The results of FECRT indicate BZ-resistance on both farms; FECR of 86% and 61%, an ERP of only two weeks and the EC50 for the LDA of 0.3 and 0.7 µg/ml, respectively. While MLs were still effective in all cases with a FECR >95% and the EC50 for the LMIA ranging from 0.06 to 0.38 µg/ml the ERP ranged from only 4 to 10 weeks. Overall the results from this study indicate that BZ was ineffective but both ivermectin and moxidectin are still effective in all groups. However, the reduced ERP results for the MLs would suggest that these products are less effective compared to label claims - a shortened ERP is believed to be an early indicator of resistance.